A detector receives echoes from the front and back surfaces of a metal plate. The second echo arrives 12 μs12\ \mu\mathrm{s}12 μs after the first. Sound travels at 5000 m/s in the plate. What is the plate thickness?
30 mm30\ \mathrm{mm}30 mm
120 mm120\ \mathrm{mm}120 mm
60 mm60\ \mathrm{mm}60 mm
15 mm15\ \mathrm{mm}15 mm